Electrical heating elements

ABSTRACT

A method and apparatus for improving the &#39;&#39;&#39;&#39;apparent&#39;&#39;&#39;&#39; resistance of an electrical heating element of the type comprising an axial resistance element of the type comprising an axial resistance element insulated from a conducting sheath by a refractory material wherein the refractory material is subjected to an electrolysing D.C. potential applied between the element and the sheath.

United States Patent Hutchinson et al. A

ELECTRICAL HEATING ELEMENTS Inventors: Eric Hutchinson, Wideopen, Newcastle upon Tyne; Gordon Hetherington, North Shields, both of England v Assignee: Thermal Syndicate Limited, Wallsend, Northumberland, England Filed: Oct. 13, 1970 Appl'. No.: 80,447

Foreign Application Priority Data Oct. 16, 1969 Great Britain ..5088 9/69 US. Cl. ..2l9/546, 219/433, 219/497,

Int. Cl. ..H05b 3/02 Field of Setirch .317/10; 338/226; 219/544, 546,

1 1 Oct. 17, 1972 [56] References Cited UNITED STATES PATENTS 2,921,174 l/l960 Welch ..2l9/492 X 2,686,250 8/1954 Schroeder .....219/433 Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E. Bell Att0mey-Curtis, Morris & Safford [57] ABSTRACT 18 Claims, 5 Drawing Figures ELECTRICAL HEATING ELEMENTS This inventionrelates to a method of and-an apparatus for improving the performance of electrical heating elements of the type comprising an axial re sistance element insulated from a conducting sheath by a refractory material (e.g. magnesia). Coaxial electric heating elements of the type described above are widely used in domestic and industrial appliances (e.g. in spiral form as heating rings for domestic cookers). Under operating conditions, the sheath is earthed and an AC voltage (typically between 120 and 250 volts. 50 or 60 Hz) is applied across the resistance element. The refractory material surrounding the resistance element is required to transmit thermal energy as'efficiently as possible to the outer sheath, but at the same time electrically to insulate the resistance element as effectively as possible from the'earthed sheath.

It is desirable'to make the thermal capacity of a heating element of the type described as small as possible. This allows the element to heat up (or cool down) in the shortest possible time. Small thermal capacity suggests as small a thickness of refractory material as possible. On the other hand, it is essential that the leakagecurrent flowing between the resistance element and the sheath during operation of the heating element (i.e. when the refractory material is at its operating temperature) should be as low as possible and this sug' gests as large a thickness of refractory material as possible.

Clearly these two considerations, of flexibility of operation and safety, conflict and the actual thickness of refractory material used in an electric heating element of the type described is a compromise. In practice, the refractory material is made thick enough to reduce the leakage current to an allowable limit and a somewhat slower response time of the heating element is accepted as inevitable.

We have now found that the electrical insulating pro perties of some refractory materials (e.g. inorganic oxide materials) can be noticeablyimproved by D.C. electrolysis and that with this improvement in insulating properties, heating elements of the type described can be satisfactorily built with thinner layers of refractory material than hitherto thought possible and yet still maintain the leakage current at an acceptable level. The method of the invention therefore leads to improved heating elements which have wide applications.

According to one aspect of the present invention, a method for improving the performance of an electric heating element comprising an axial resistance element insulated from a conducting sheath by a layer of refractory material, comprises periodically applying a D.C. potential between the sheath and the resistance element.

The actual D.C. voltage applied andthe periodicity of its application can be optimized empirically for any particular refractory material. A D.C. voltage of between 5 and 20 percent (conveniently around percent) of the A.C. voltage has been found to be convenient in many cases. The D.C. voltage should not be less than 5 volts. We prefer to apply the electrolysing D.C. voltage at fairly regular periods throughout the operation of the heating element and conveniently the ratio of the total periods during which the D.C. potential is applied and the total period during which it is not applied lies between the limits 30:70 and 1:99. An on time of some 5 percent and an off time of some 95 percent (Le. a ratio of 5:95) has been found suitable. It must be stressed, however, that wide variations in the magnitude and periodicity of application of the D.C. voltage are envisaged. If one considers the normal leakage current of a well-used 240 volt AC heating element at operating temperature as being unity then in. a typical case, a halving of leakage current can be obtained by applying 30 volts D.C. between the resistance element and the sheath for betweenv one and two minutes.' When the D.C. voltage is removed, the leakage current slowly rises to its original value but in a case such as that being considered, one would expect it to be at least 10 minutes after the electrolysing voltage had been removed before the leakage current had returned to percent of its original value. Generally I speaking, it appears that the reduction 'in leakage current varies exponentially with the time of application of the D.C. voltage, themost rapid rate of reduction occurring in the early stages of electrolysis. Very approximately, the return of the, heating element to normal conditions after the D.C. voltage has been removed, can be considered to be linear.

According to a further aspect of the present invention, apparatus for-improving the performance of an electric heating element comprising an axial resistance element insulated from a conducting sheath by a layer of refractory material-comprises first means for applying an A.C. voltage between the resistance element and the sheath and second means for periodically applying a D.C. voltage between the sheath and the resistance element.

One embodiment of apparatus in accordance with the invention would be a domestic electric cooking stove.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1 and 2 show circuit arrangements for periodi- Ycally applying a D.C. electrolysing voltage to the FIGS. 1 and 2 illustrate two simple methods of apply- I ing a periodic electrolysing D.C. voltage between the resistance element and the sheath of a heating element. In FIG. 1, a switch S is operated cyclically. With the switch S closed, a heating element 1 heats up in the normal way from an A.C. supply 2, one side of which should be at, or near to earth potential. When the switch S is open, D.C. current flows via a diode D, a current limiting resistor -R and the insulating material interposed between the resistance element and the outer earthed sheath of the heating element 1.

A radiant element on an electric domestic cooker normally employs a cyclic switch for heat control purposes and this switch can simply (and relatively cheaply) be modified to the circuit shown in FIG. 1.

FIG. 2 shows a modified arrangement in which the heating element 1 is energized from the A.C. supply 2 via a bi-directional thyristor T. The thyristor T is asymmetrically fired by virtue of a firing delay introduced by a Zener diode D1 in circuit with a variable resistor R1. The resultant wave form fed to the heating element 1 is a distorted sinusoidal wave form, the distortions being different in the positive half cycles as compared to the distortion in the negative half cycles. In consequence, the resultant wave form may be considered as a combination of an A.C. voltage and a D.C. bias. The D.C. bias (whose magnitude can be controlled by varying the degree of asymmetry introduced into the output wave form by the delayed firing of the heating element 1) can be controlled by the variable resistor R1 since this resistor is controlling the firing angle of the thyristor T.

It will be appreciated that the resistor R1, by controlling the instant in each half cycle whenthe thyristor T is fired, controls the energy supplied to the heating element 1 and thus serves to control the temperature attained by the element.

Clearly the controlled circuit shown in FIG. 2 is more complicated, than that commonly employed for the heating elements of a domestic electric cooker, but it will be appreciated that the circuit shown acts as an energy regulator and does permit very smooth control over element temperature. Further, by virtue of the application of a D.C. electrolysing voltage, the electrical resistance of the insulating material in the heating element 1 has an improved performance which enables thinner layers of such material to be used and thus reduces the cost and improves the efficiency of the heating element.

Referring to the graphs, FIG. 3 shows the variation of A.C. leakage current with the time of application of the D.C. electrolysing voltage. The ordinate shown in FIG. 3 expresses the A.C. leakage current immediately after removal of the D.C. electrolysing voltage as a ratio of the A.C. leakage current existing in an unelectrolysed heating element. The abscissa in FIG. 3 shows the time (in minutes) for which the D.c. electrolysing voltage is applied and it will be seen that an approximately exponential type curve is obtained indicating that only marginal improvements are obtained if the D.C. electrolysis is continued for more than about five minutes. In the production of FIG. 3 the D.C. voltage was 30 volts and the A.C. voltage was 240 volts. The heating element was operated so that the sheath temperature was 850 C. and the insulating material was magnesium oxide.

FIG. 4 shows how the effect of the D.C. electrolysis decays with time after the electrolysing voltage has been removed. As in FIG. 3, the ordinate shows the ratio of A.C. leakage currents before and after application of the D.C. voltage and the abscissa shows the time (in minutes) from the termination of the electrolysis. From the graph it will be seen that a substantially linear relationship exists, the leakage current steadily returning to the value pertaining prior to the application of the electrolysing voltage. As in the previous graph, the results shown in FIG. 4 were obtained using a 30 volt D.C. electrolysing voltage, 240 volts A.C. heating voltage and a heating element, with magnesium oxide insulating material, operating at a sheath temperature of 850 C.

FIG. 5 shows a plot of leakage currents (ordinate) in milliamps against time in hours as abscissa for a heating element which was operated normally prior to the point X and was then subjected to D.C. electrolysis in the manner described (and using a circuit as shown in FIG. 1) the A.C. voltage being applied for 97 seconds in each hundred and D.C. voltage for the last three seconds.

From the graph it will be seen that after the D.C. electrolysing voltage cycle was commenced the leakage current rapidly fell and remained at this reduced value until the cycling was stopped at time Y (i.e. for a while no further D.C. electrolysing voltages were applied). Oncessation of the D.C. electrolysis, the leakage current rapidly returned to a value approaching that existing originally but rapidly fell again when the D.C. voltage cycling was restored at point Z. The ripple shown on the graph has been established to be due to variations in the A.C. voltage supply.

We have noticed that the advantages obtained by D.C. electrolysis are only apparent in the case of heating elements which have been operated at high temperatures for a considerable time. It is possible that metallic ions originating from the resistance element and the outer sheath migrate into the refractory insulating material and reduce its effective resistance. It is possible that the periodic application of a D.C. voltage has the effect of sweeping these impurity ions to the negative electrode of the D.C. circuit, thus effecting a temporary improvement in the resistance of the insulating layer.

What is claimed is:

1. A method of effecting a significant practical reduction in the overall leakage current with time occurring across a refractory material electrically insulating an axial resistance element from a surrounding electrically conducting sheath in a heating element, comprising the steps of heating said element by applying a A.C. voltage thereacross, periodically applying a unidirectional D.C. potential between the sheath and the resistance element of a magnitude and for a time sufficient to effect said reduction.

2. A method as claimed in claim 1 wherein in said step of periodically applying a D.C. potential the sheath and the resistance element, the D.C. potential used is between 5 and 20 percent of the A.C. voltage applied.

3. A method as claimed in claim 1 in said step the D.C. voltage applied is at least 5 volts.

4. A method as claimed in claim 3, wherein in said step of periodically applying a D.C. potential the ratio of the total period during which the D.C. potential is applied to the total period during which it is not applied lies between the limits 30:70 and 1:99.

5. A method as claimed in claim 4 the said ratio in said step is about 5:95.

6. A method as claimed in claim 4 wherein said step each period during which the D.C. potential is applied is less than 2 minutes.

7. A method as claimed in claim 6 wherein said step the D.C. potential is applied for a few seconds in each minute.

8. A method as claimed in claim 1 wherein in said step of periodically applying a D.C. potential between the sheath and the resistance element, the D.C. potential used is above five per cent of the A.C. voltage applied.

9. A method as claimed in claim 8, wherein in said step of periodically applying a D.C. potential the ratio of the total period during which the D.C. potential is applied to the total period during which it is not applied lies between the limits 30:70 and 1:99.

10. A method as claimed in claim 9 the said ratio in said step is about 5:95.

1 l. A method as claimed in claim 9 wherein said step each period during which the D.C. potential is applied is less than 2 minutes.

12. A method as claimed in claim 11 wherein said step the D.C. potential is applied for a few seconds in each minute;

13. An electric heating apparatus having a heating 7 element with an axial resistance element insulated from a grounded conducting sheath by a layer of a refractory material, a device for increasing the effective resistance of said refractory material comprising means applying an AC. voltage across the resistance element, second means applying between the sheath and the resistance element a unidirectional D.C. voltage of a magnitude and for a time sufficient to achieve a practical significant increase in the effective resistance of said refractory material, and third means for periodically interrupting said D.C. voltage.

14. A device as claimed in claim 13, in which the second means comprises a series-connected diode and resistor disposed in parallel with a switch serving to disconnect the AC. voltage supply from the heating element.

15. A device as claimed in claim 13, in which the second means is positioned in series with said axial re- 18. A device as claimed in claim 13 wherein the D.C. voltage applied by 'said second means is at least 5 volts. 

1. A method of effecting a significant practical reduction in the overall leakage current with time occurring across a refractory material electrically insulating an axial resistance element from a surrounding electrically conducting sheath in a heating element, comprising the steps of heating said element by applying a A.C. voltage thereacross, periodically applying a unidirectional D.C. potential between the sheath and the resistance element of a magnitude and for a time sufficient to effect said reduction.
 2. A method as claimed in claim 1 wherein in said step of periodically applying a D.C. potential the sheath and the resistance element, the D.C. potential used is between 5 and 20 percent of the A.C. voltage applied.
 3. A method as claimed in claim 1 in said step the D.C. voltage applied is at least 5 volts.
 4. A method as claimed in claim 3, wherein in said step of periodically applying a D.C. potential the ratio of the total period during which the D.C. potential is applied to the total period during which it is not applied lies between the limits 30: 70 and 1:99.
 5. A method as claimed in claim 4 the said ratio in said step is about 5:95.
 6. A method as claimed in claim 4 wherein said step each period during which the D.C. potential is applied is less than 2 minutes.
 7. A method as claimed in claim 6 wherein said step the D.C. potential is applied for a few seconds in each minute.
 8. A method as claimed in claim 1 wherein in said step of periodically applying a D.C. potential between the sheath and the resistance element, the D.C. potential used is above five per cent of the A.C. voltage applied.
 9. A method as claimed in claim 8, wherein in said step of periodically applying a D.C. potential the ratio of the total period during which the D.C. potential is applied to the total period during which it is not applied lies between the limits 30: 70 and 1:99.
 10. A method as claimed in claim 9 the said ratio in said step is about 5:95.
 11. A method as claimed in claim 9 wherein said step each period during which the D.C. potential is applied is less than 2 minutes.
 12. A method as claimed in claim 11 wherein said step the D.C. potential is applied for a few seconds in each minute.
 13. An electric heating apparatus having a heating element with an axial resistance element insulated from a grounded conducting sheath by a layer of a refractory material, a device for increasing the effective resistance of said refractory material comprising means applying an A.C. voltage across the resistance element, second means applying between the sheath and the resistance element a unidirectional D.C. voltage of a magnitude and for a time sufficient to achieve a practical significant increase in the effective resistance of said refractory material, and third means for periodically interrupting said D.C. voltage.
 14. A device as claimed in claim 13, in which the second means comprises a series-connected diode and resistor disposed in parallel with a switch serving to disconnect the A.C. voltage supply from the heating element.
 15. A device as claimed in claim 13, in which the second means is positioned in series with said axial resistance.
 16. A device as claimed in claim 13, in which the second means comprises a thyristor-type device in parallel with a Zener-diode type device plus a capacitor with the control of the thyristor-type device connecting to the junction between the capacitor and the Zener-diode type device.
 17. A device as claimed in claim 13 wherein the D.C. voltage applied by said second means is five per cent above the A.C. voltage applied by the first means.
 18. A device as claimed in claim 13 wherein the D.C. voltage applied by said second means is at least 5 volts. 